Raised Beds

Raised Beds

Hi everyone. I'm thinking about trying my hand at making a raised bed this weekend. I got a book that has simple instructions on how to make one but I still have a few questions. Do I have to put a mat down or some type of tarp so that the new soil I buy doesn't interact with the soil that's in the yard which isn't all that great? Also I've read beds show be at least 12 inches deep. If I put a barrier on the ground under it is 12 inches enough for roots to grow in? I'm thinking of putting a few veggies in it, probably a squash plant and a tomatoe and the rest would be hummingbird attracting plants like Salvia and Foxglove.
Pictures – just because

Pictures – just because

Castor bean and trumpet plant

Castor bean and trumpet plant

Tomato question, and pictures

Tomato question, and pictures

Bugs, illness or Martian heads?

Bugs, illness or Martian heads?

Oh noes! Garden pests!

Oh noes! Garden pests!

pics/plant ID

pics/plant ID